Various attempts have been made to provide sensing capabilities in the context of petroleum exploration, production, and monitoring, with varying degrees of success.
Recently, these attempts have included the use of fiber optic cables to detect acoustic energy. Because the cables typically comprise optically conducting fiber containing a plurality of backscattering inhomogeneities along the length of the fiber, such systems allow the distributed measurement of axial strain along an optical fiber by measuring backscattered light from a laser pulse input into the fiber. Because they allow distributed sensing, such systems may be referred to as “distributed acoustic sensing” or “DAS” systems.
DAS systems operate using principles similar to Optical Time-Domain Reflectometry (OTDR). In OTDR, a fiber-optic cable is probed with a laser pulse from an interrogation unit. Defects in the glass backscatter the pulse (Rayleigh scattering) as it propagates along the fiber and the backscattered photons are received in a photodetector. The data is used to map the reflectivity of the fiber along its length. In DAS, external acoustic disturbances modulate the backscattered light from certain sections of the fiber. By recording these traces at high data rates (−5 kHz), DAS transforms the fiber into a large number of distributed microphones or sensors.
One use of DAS systems is in seismic applications, in which seismic sources at known locations transmit acoustic signals into the formation, and/or passive seismic sources emit acoustic energy. The signals are received at seismic sensors after passing through and/or reflecting through the formation. The received signals can be processed to give information about the formation through which they passed. This technology can be used to record a variety of seismic information.
While there exists a variety of commercially available DAS systems that have varying sensitivity, dynamic range, spatial resolution, linearity, etc., these systems tend to have an undesirably low ratio of signal to noise. This is due in part to the nature of back-scattering measurements, which rely on the presence of reflectors along the length of the fiber to provide the distributed sensing.
In addition, it has been discovered that distributed acoustic systems, particularly those that rely on Rayleigh backscattering are subject to significant noise and that the noise is random or statistical in nature. In instances where a high degree of precision is required, such as when a distributed acoustic system is used to monitor seismic signals, the signal to noise ratio is so small as to significantly reduce the value of the sensing system.
One way to increase the signal to noise ratio is to fire multiple light pulses at different frequencies and wavelengths into the fiber. This technique is disadvantageous, however, because it requires more complex data processing and optical components than a single-pulse system.
Thus, it is desirable to improve the signal to noise ratio of a DAS system while maintaining the relatively low cost and simplicity of DAS hardware.